Core-shell Ag@SiO @mSiO mesoporous nanocarriers for metal ... · Core-shell Ag@SiO2@mSiO2...
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Core-shell Ag@SiO2@mSiO2 mesoporous nanocarriers for
metal-enhanced fluorescence
Jianping Yang a, Fan Zhang a*, Yiran Chen a, Sheng Qian a, Pan Hu a, Wei
Li a, Yonghui Deng a, Yin Fang a, Lu Han a, Mohammad Luqman b,
Dongyuan Zhao a*
a Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Key Laboratory of Molecular Engineering of Polymers of the
Chinese Ministry of Education, Laboratory of Advanced Materials, Fudan University,
Shanghai 200433, P. R. China
b Chemical Engineering Department, College of Engineering, King Saud University,
Kingdom of Saudi Arabia
Email: [email protected], [email protected]
Tel: 86-21-5163-0205; Fax: 86-21-5163-0307
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Experimental Section
Chemicals. All chemicals were of analytical grade and used without further
purification. AgNO3, ethylene glycol, ammonia aqueous solution (28 wt %), NaCl,
acetone, NH4NO3, tetraethyl orthosilicate (TEOS) and hexadecyltrimethylammonium
bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co. (China).
Polyvinylpyrrolidone (PVP, Mw = 55000), Eosin isothiocyanate (EiTC), Fluorescein
isothiocyanate (FiTC), Rhodamine B (Rh B) and poly(allylamine hydrochloride)
(PAH, Mw = 56000) were obtained from Sigma Aldrich. Deionized water was used in
all experiments.
Synthesis of Ag nanoparticles. Ag nanoparticles with diameter about 50 nm were
synthesized in large scale via a modified method reported by Xia et al.1 2.5 g of PVP
(Mw = 55000) was dissolved in 200 mL of ethylene glycol before 0.5 g of AgNO3
was added. After the three-neck flask was settled in an oil bath, the mixture was then
heated to ~ 130 °C within 25 min under vigorous stirring and maintained at 130 °C
for 1 h to obtain the Ag nanoparticles. The nanoparticles were isolated by
precipitating the solution with acetone (800 mL), followed by centrifugation at 10000
rpm for 3 min, and re-dispersed in 4 mL of ethanol to obtain the 0.05 g (Ag
nanoparticles)/mL solution.
Synthesis of Ag@SiO2 core-shell particles. The Ag@SiO2 particles with different
silica thickness were prepared according to Stöber method. In a typical procedure for
the silica layer coating with the thickness of ~ 3 nm, 2 mL of Ag nanoparticle/ethanol
solution (0.05 g/mL) obtained above was dispersed in the mixture of ethanol (80 mL)
and water (20 mL) and 1 mL of ammonia aqueous solution (28 wt%) under stirring,
then 15 μL of TEOS was added slowly with continuous stirring for 5 sec. The reaction
was continued for 6 h. The Ag@SiO2 particles was separated by centrifugation and
washed by ethanol and water for several times. The thickness of the silica coating
layer could be increased gradually with increasing the TEOS concentration. For
example, silica coating layer with the thickness of ~ 8 nm was obtained using the
procedure similar to the above increasing the TEOS concentration to 60 μL.
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Synthesis of Ag@SiO2@mSiO2 core-shell particles. The Ag@SiO2@mSiO2
core-shell particles were prepared through a surfactant-templating sol-gel approach by
using CTAB as a template. In brief, the above synthesized Ag@SiO2 particles were
added into the solution containing 25 mL of water, 15 mL of ethanol, 75 mg of CTAB
and 0.25 mL of ammonia aqueous solution (28 wt %). The mixture was turned to
homo-dispersed solution after being agitated ultrasonically and mechanically for 30
min each. It was followed by the addition of 120 μL of TEOS was added dropwisely
with continuous stirring for about 10 seconds and the reaction was continued for 6 h.
The particles were collected by centrifugation and washed with ethanol and water,
respectively. The CTAB surfactant was removed by solvent extraction method using
60 mL of NH4NO3/ethanol solution (6 g/L) and refluxed at 60 °C for 1 h. This
extraction process was repeated twice. After centrifugating and washing with ethanol
and water, the Ag@SiO2@mSiO2 core-shell nanocarriers were obtained.
Loading of fluorophores in mesoporous silica shells. The dyes including EiTC,
FiTC and Rh B were loaded in the mesoporous silica shells via an impregnation
method. For example, 12 mg of Ag@SiO2@mSiO2 core-shell particles with SiO2
spacer of 8 nm were re-dispersed in 6 mL of ethanol. Brown vials were loaded with
500 μL of Ag@SiO2@mSiO2/ethanol solution and 500 μL of ethanol, followed by
addition of 5, 10, 15, 20 μL of EiTC/ethanol solution (0.5 mg/mL), respectively. After
being stirred for 24 h, 500 μL PAH (2 mg/mL) ethanol solution was injected and kept
stirring for 3 h. The products were collected by centrifugation and washed with
ethanol for 3 times. Finally, the products were re-dispersed in 4 mL ethanol to obtain
the 0.25 mg/mL solution. FiTC and Rh B were loaded in the mesopore channels of the
Ag@SiO2@mSiO2 nanocarrier using the same procedure as that of EiTC.
Loading of FiTC-EiTC in the mesoporous silica shells. 500 μL of the
Ag@SiO2@mSiO2 ethanol solution (2 mg/mL) with SiO2 spacer of 8 nm was diluted
with 500 μL of ethanol. 15 μL of EiTC/ethanol solution (0.5 mg/mL) was injected and
stirred for 24 h, followed by adding different amount of 3, 6, 9, 12 and 15 μL of FiTC
(0.5 mg/mL) was added and continuous stirring for another 6 h, respectively. 500 μL
of PAH (2 mg/mL) ethanol solution was injected and kept stirring for 3 h. The
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particles were separated by centrifugation and washed with ethanol, and then diluted
to 4 mL with ethanol (0.25 mg/mL) for use.
Dissolving Ag core from Ag@SiO2@mSiO2 (control sample). The Ag cores in the
core-shell structured Ag@SiO2@mSiO2 particles could be removed by NaCl
solution.2-3 For example, 200 μL of the above-mentioned EiTC-, FiTC-, Rh B- and
EiTC/FiTC-loaded Ag@SiO2@mSiO2 composite particles in ethanol solution (0.25
mg/mL) were dispersed into 3.8 mL of NaCl solution (250 mM) and kept stirring for
one day, respectively. Thus the hollow particles without Ag cores (control sample)
were obtained without washing and centrifugation. The corresponding compared
Ag@SiO2@mSiO2 nanocarrier was also diluted with 3.8 mL water to ensure
concentration of dyes in the Ag@SiO2@mSiO2 nanocarrier and control sample are the
same.
Characterization. Transmission electron microscopy (TEM) measurements were
carried out on a JEOL 2011 microscope (Japan) operated at 200 kV. All samples were
first dispersed in ethanol and then collected using copper grids covered with carbon
films for measurements. Scanning electron microscopic (SEM) images were obtained
on a Philip XL30 microscope (Germany). A thin film of gold was sprayed on the
sample before this characterization. Field-emission scanning electron microscopy
(FESEM) images were obtained on a Hitachi S-4800 microscope (Japan). Powder
X-ray diffraction (XRD) patterns were recorded on a Bruker D4 X-ray diffractometer
(Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Nitrogen sorption
isotherms were measured at 77 K with a Micromeritcs Tristar 3000 analyzer (USA).
Before measurements, the samples were degassed under vacuum at 180 °C for at least
6 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific
surface areas (SBET), using adsorption data in a relative pressure range from 0.04 to
0.2. The pore volume and pore size distributions were derived from the adsorption
branches of isotherms using Barrett-Joyner-Halenda (BJH) model. The total pore
volume, Vt, was estimated from the amount adsorbed at a relative pressure P/P0 of
0.995. Fluorescence spectra were recorded on an F-4500 spectrofluorometer (Hitachi
High-Technologies). FiTC, EiTC and Rh B were excited at 485, 520 and 540 nm,
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respectively. The bandpass was set at 5 nm both excitation and emission, scan speed
at 2400 nm/min and PMT voltage at 700 V for all measurements. UV-Vis absorption
spectra were measured on a Jasco spectrophotometer (V-550) (Japan). Confocal
luminescence images were made with an Olympus FV1000 (Japan), with λex = 515 nm
as the excitation source and emissions were collected in the range of λ = 535 – 555
nm. All the measurements were carried our in the same condition.
References
1. Y. G. Sun and Y. N. Xia, J. Am. Chem. Soc., 2004, 126, 3892.
2. M. L.-Viger, M. Rioux, L. Rainville and D. Boudreau, Nano Lett., 2009, 9, 3066.
3. M. L.-Viger, D. Brouard and D. Boudreau, J. Phys. Chem. C, 2011, 115, 2974.
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Fig. S1 SEM images (a, c, e) and FESEM images (b, d, f) of the Ag nanoparticles (a,
b), the core-shell Ag@SiO2@mSiO2 nanocarrier with the silica spacer in the thickness
of 3 nm (c, d) and 8 nm (e, f).
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Fig. S2 TEM image of the core-shell Ag@SiO2@mSiO2 nanocarrier with the SiO2
spacer in the thickness of 3 nm (left). Powder XRD patterns (right) of the
Ag@SiO2@mSiO2 nanocarrier with the silica spacer in the thickness of 3 nm (a) and
8 nm (b).
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Fig. S3 Absorption spectra of (a) EiTC-loaded Ag@SiO2@mSiO2 nanocarrier and (b)
after dissolving the silver nanoparticles (inset). The thickness of the silica spacer is 8
nm and the concentration of EiTC is 10.5 x 10-6 mol/L.
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Fig. S4 Fluorescence emission spectra of the FiTC-loaded Ag@SiO2@mSiO2
nanocarrier with the silica-spacer in the thickness of 3 nm (a) and 8 nm (b), in which
the FiTC concentration increases from 6.4 x 10-6 to 19.2 x 10-6 mol/L. Fluorescence
spectra of the FiTC-loaded Ag@SiO2@mSiO2 nanocarrier with the silica spacer
thickness of 3 nm (c) and 8 nm (d) and after dissolving the silver nanoparticles. The
excited wavelength is 485 nm.
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Fig. S5 Fluorescence emission spectra of the RhB-loaded Ag@SiO2@mSiO2 with the
silica spacer in the thickness of 3 nm (a) and 8 nm (b), in which the RhB
concentration increases from 5.2 x 10-6 to 20.8 x 10-6 mol/L. Fluorescence spectra of
the RhB-loaded Ag@SiO2@mSiO2 nanocarrier with the silica spacer thickness of 3
nm (c) and 8 nm (d) and after dissolving the silver nanoparticles. The excited
wavelength is 540 nm.
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Fig. S6 Fluorescence spectra of (a) FiTC (donor)/EiTC (acceptor) (red line) and EiTC
(acceptor)-only (black line) loaded in Ag@SiO2@mSiO2 nanocarrier, (b) FiTC
(donor)/EiTC (acceptor) loaded Ag@SiO2@mSiO2 nanocarrier before (red line) and
after (black line) dissolving the silver nanoparticles. The loading amounts of EiTC
and FiTC are 10.5 x 10-6 and 19.2 x 10-6 mol/L, respectively. Fluorescence spectra of
FiTC (donor)/EiTC (acceptor) loaded Ag@SiO2@mSiO2 nanocarrier before (c) and
after (d) dissolving the silver nanoparticles, while the concentration of EiTC (acceptor)
is kept constant at 10.5 x 10-6 mol/L. The thickness of the silica-spacer layer is 8 nm
and the excitation wavelength is 485 nm for (a-d).
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